Foul Ole Ron's Smell
The Smell is the name of the odour that usually accompanies the beggar Foul Ole Ron wherever he goes. It is so horrible that most noses simply shut down in the presence of the Smell, though one could tell Ron is nearby simply by how their ear wax starts melting out of their ears.
Even a small quantity of oil of Scallatine, which is so pungent that getting a drop of it on your hands will never wash off, is no match for the Smell, which could effectively wear it as a deodorant. As unfortunate as professional connoisseur of smells Angua von Uberwald was to encounter a concoction dropped by William de Worde to throw her off his trail, it could have been far worse.
In fact, the Smell is such a defining characteristic of Foul Ole Ron that it has acquired sentience. It regularly pays visits to the art gallery and opera, and tends to arrive at a destination before Ron does. Ron is actually outclassed by his Smell. The Smell is not stupid; when Lord Vetinari advised it "Do not let me detain you," it found it had business elsewhere.